1. Field of the Invention
The present invention relates to a method of joining metallic members such as electric or electronic parts together, using a conductive binder containing no lead, and to the joined metallic members, and more particularly, to a method of joining electric or electronic parts including circuit substrates together, using a solder paste composed of a tin and lead solder (Sn--Pb solder) and to the joined electric or electronic parts.
2. Related Art
Soldering is a technique of joining physical objects each other using a material with relatively low melting point and has been used since olden times. The origin of soldering is said to be able to go back to ancient Mesopotamian civilization. In modern industry, soldering is widely used in joining and mounting electronic devices. For example, in mounted substrates, soldering is used for joining and the like of mounting electronic parts of semiconductors, microprocessors, memories, resistors and others on substrates. Advantages of soldering reside in not only fixing of parts on substrates but also formation of electric connections by the conductivity of metals contained in the solder. In this point, solder is different from organic adhesives.
Solder generally used is an eutectic solder composed of tin and lead, the theoretical eutectic point of which is 183.degree. C., and it is used in joining substrates and others. The tin and lead eutectic solder has the feature that it does not damage printed substrates and others with heat, because its eutectic point is lower than the temperatures that most of the thermosetting resins begin to gasify. Moreover, the eutectic solder is known that its tin component forms a peculiar metal compound layer in the interface of a copper plate to strengthen the adhesive strength between the solder and the copper plate more.
The eutectic solder composed of tin and lead with such features is important in joining and mounting parts in the manufacture of electronic devices. In the fine pitch soldering treatment which is required for thick film formation, conductive circuit formation, semiconductor mounting and others, a screen printing method or the like with solder paste, which is prepared by mixing a solder powder and a flux to form a paste, is used. Consequently, the demand of solder is more and more increasing in the technology of mounting electronic parts, with the rapid spread of personal devices represented by personal computers, cellular phones, pagers and the like.
The spread of electronic devices has made people's life comfortable. On the other hand, however, it is true that waste electronic devices are thrown away in large quantities, and it is feared that such waste may pollute the environment. For this reason, the recycle use of the waste and the manufacturing processes without the use of highly harmful materials have been proposed. In particular, the elimination of highly harmful materials is desirable in view of the prevention of the environmental pollution, and the jointing technology with soldering is also considered to be developed more.
Although the tin and lead eutectic solder has a characteristic that its wetting property to base materials is better than those of other metal mixtures, lead contained in the above solder is feared that in a case of the landfill of thrown electronic devices, their exposure to acid rain for a long time may elute lead ion into the soil to cause a problem of poisoning. To solve this problem, the technique of immobilizing lead is proposed, but no sufficient data extended for a long period of time are obtained about the diffusion of lead into the soil. Moreover, according as memory elements have become to be high-density in recent years, the damage of electronic devices due to radiation (.alpha.ray) from lead has been highlighted and it is required to reconsider the use of lead in the respect of coping with high-density mounting in semiconductor devices.
In the above circumstances, joining techniques with solder containing no lead are required. However, the solder in which lead is replaced with other metal and the solder composed of a combination of other metals are very inferior in wetting property and do not show enough joining property. For example, although the use of various kinds of solder composed of tin and zinc, tin and silver, and others has been tried, their wetting property is inferior to the solder composed of tin and lead. Furthermore, solder containing silver is difficult to be used in large quantities as a product for wide use because silver itself is a noble metal and so its applications are easily limited into special regions.
In joining and mounting electric and electronic parts in present situation, because devices and equipment for a screen printing method with solder paste have been spread in manufacturing sites, solder paste of solder not containing lead is desired to be put to practical use. However, at present, the practical use of solder paste in a complicated system like tin, silver and bismuth solder is only barely tried, and part mount using solder paste without lead has been scarcely put to practical use. As for this reason, it is also considered that a part fabrication process or the like can not be realized economically using such a method that a special atmosphere or environment is necessary for working surroundings.